The following abbreviations are herewith defined.    ADC analog-to-digital converter    AM amplitude modulation    ASIC application specific integrated circuit    BB baseband    BER bit error rate    BLER block error rate    CDMA code division multiple access    CPU central processing unit    CRC cyclic redundancy check    DPCH dedicated physical channel    DS-CDMA direct sequence CDMA    DSP digital signal processing    Ec/Io code power-to-in-band interference ratio    EVM error vector magnitude    FDD frequency division duplexing    FPGA field programmable gate array    IC integrated circuit    ICP input compression point    IF intermediate frequency    IIP2 second-order input intercept point    IIP3 third-order input intercept point    IMD2 second-order intermodulation product    IMD3 third-order intermodulation product    ISI intersymbol interference    LNA low noise amplifier    LO local oscillator    MDS minimum detectable signal    MS mobile station    NF noise figure    QoS quality of service    RX receiver    RF radio frequency    RSS received signal strength    SIR signal-to-interference ratio    SNR signal-to-noise ratio    TX transmitter    VCO voltage controlled oscillator    WCDMA wide bandwidth CDMA    3G third-generation (cellular communications system)
The dynamic range requirements of a radio receiver are normally defined by the system specifications assuming worst-case operational conditions. However, the worst case conditions are rarely encountered during the typical operation of the receiver. In general, the strength of the received signal and any interfering signals depends on the distance from the transmitter and on the particular radio channel, including fading and other effects.
Substantially all radio receivers in mobile terminals, such as cellular telephones and other types of mobile receivers use some type of automatic gain control mechanism in the receiver for compensating for dynamically changing reception conditions. The total gain of the receiver is adjusted to the desired level for the received signal detector or analog-to-digital converter (ADC) using either analog or digital gain control signals. These control signals steer the gain of the RF, baseband and possibly IF blocks. The gain is typically set by the value of the received signal strength (RSS) at the received radio channel, or by the total signal strength at the input of the ADC, using some specific algorithm. The gain control can be also based on the level at the ADC input if a part of the channel filtering or despreading in a CDMA system is performed in the digital domain. All of these techniques are well-known and are employed in many cellular receivers.
In addition to gain control, more sophisticated control methods have also been presented for radio reception under dynamically changing conditions.
In general, the trade-off between power consumption and dynamic range can be utilized to minimize the power consumption at each moment of time. Also, the modularity of base station applications could benefit from the use of a modular design. Often these techniques control the biasing current or supply voltage of one or several receiver blocks. Referring also to FIG. 1 there are shown various prior art techniques for implementing adaptive reception in a receiver. These include adjusting the biasing current to a device 1 (FIG. 1A), adjusting the supply voltage of the device 1 (FIG. 1B), bypassing a stage (FIG. 1C), switching between stages (FIG. 1D), and switchable feedback (FIG. 1E). The power consumption can thus be scaled in various ways, such as by adjusting the bias current as in FIG. 1A, or by switching between parallel stages as in FIG. 1D, or bypassing certain devices that can also be powered down (FIGS. 1B and 1C). The controlled device 1 can be a single transistor, an amplifier, a mixer, a filter or any other active single component or multiple component circuit block in a radio receiver.
Reference in this regard can be made to, for example, U.S. Pat. Nos. 5,179,724, 6,026,288 and 5,697,081, as well as to WO97/41643, WO00/18023 and EP0999649A2.
Overall control is normally based on one or several measured parameters. These include the received signal strength (RSS), the signal-to-interference ratio (SIR) (or its estimate at the detector), Ec/Io in CDMA systems (see U.S. Pat. No. 5,940,749, WO00/18023) and the total power at RF, IF or baseband (see WO97/41643). Also, interfering signals can be estimated by measuring neighboring channels at separate moments of time utilizing the same circuitry as the received signal (see EP0999649A2). Intermodulation can be estimated separately by switching a controllable attenuator into the signal path (see, for example, U.S. Pat. No. 5,907,798, U.S. Pat. No. 5,909,645, U.S. Pat. No. 6,052,566 and U.S. Pat. No. 5,697,081). Also, the known transmitted power can be utilized for power scaling in a receiver in those cases where transmission and reception occur simultaneously (see, for example, U.S. Pat. No. 5,815,821, WO99/45653 and WO00/18023.)
However, in general all of these techniques exhibit as a weakness a requirement to make accurate estimates of the received signal and also the level of the total interference. Typically, the control is based on some fixed thresholds that categorize both the received signal and the interference to be either “weak” or “strong”.
One standard requirement in cellular communication systems is to measure the RSS. However, the RSS describes only the level of the received radio signal (over the channel bandwidth, for example) with a certain accuracy. It is also possible to estimate the SIR in the band of interest using well-known digital techniques, and the estimation of the SIR is currently a required measurement in some radio systems, such as in the 3G CDMA system. Unfortunately, the total interference arises from several sources, which are very difficult or impossible to distinguish from one another based on conventional digital algorithms, in particular those algorithms whose complexity would not be unreasonable to execute in a mobile station employing its local computing resources. For example, the sources of interference in a CDMA system include at least: interference from other code channels of the same base station, interference from other code channels in the same frequency band from other near-by base stations, interference from jamming signals, thermal noise in the band of interest, as well as additional noise and interference caused by the RF circuitry of the receiver itself.
The last factor, i.e., the additional noise caused by the RF receiver circuitry, includes at least a noise figure (NF) of the receiver, additional interference due to intermodulation and phase noise of the oscillators in the receiver, additional noise due to intersymbol interference (ISI) and, in digital radio systems, quantization noise. All of these are well-known phenomena in radio reception.
In full-duplex systems, where reception and transmission occur simultaneously (such as in CDMA systems), the undesired leakage of the transmitted signal into the receiver can also cause a problem. Also, some receiver architectures have their own specific problems that give rise to additional interference, such as AM-distortion in direct conversion receivers.
In any event, it should be appreciated that without intelligent logic it is practically impossible to separate these different sources of interference and to determine their relationship to the SIR. As a result, conventional radio receivers are designed to operate under the worst-case conditions by always operating at the maximum possible performance (and power consumption) level.
As was noted above, in conventional radio receivers it is known to adjust the gain according to the RSS or signal level at the ADC input. As the reception parameters typically change during operation when the gain control is applied, the power consumption is typically optimized with respect to certain parameters such as the noise figure (NF), according to the worst-case scenario. Because the total interference cannot be predicted at each moment of time, additional headroom must be made available under typical operating conditions. Practically speaking, the gain control is required in all cellular systems to extend the signal range of the desired channel at the input of the receiver. However, a variation in the gain control does not typically imply that the power consumption of the receiver is scaled accordingly.
The gain and other receiver parameters are typically controlled using logic based on the RSS and the total interference after the preselection filter, or after some other filtering stage. Hence, the decision is based on logic that does not indicate whether an out-of-band interferer will alias with a signal in the band of interest due to intermodulation. The out-of-band interferer(s) can thus be filtered out of the receive chain so that they can only degrade the performance due to intermodulation, gain compression or desensitization, such as by raising the noise level or noise floor of the receiver circuitry. Hence, the estimate is based on information that does not have a straightforward relationship to the interference in the RF band of interest. In that the decision logic is typically based simply on threshold values and thus gives, at best, only a coarse approximation of the receiving environment, the result is that certain receiver parameters can be set at levels that exceed what is required in the particular receiving environment.
Only in certain limited cases can the interferer be defined with reasonable accuracy in advance. For example, the linearity of the receiver can be increased using additional current when some known interference (normally due to TX leakage) exists in the system. In this case the logic can react only to a very limited number of conditions and, typically, the receiver performance is made significantly better than what is actually required.
In conventional approaches it is also known that interfering signals can be measured with the same receiver signal path as the actual reception signal path, but at the different moments of time. For example, in GSM there are mandatory measurements of other radio channels that can be measured, and their values can be used in the control logic. A switchable attenuator in the signal path can also be used to estimate the ratio between intermodulation and other interference sources in the band of interest, as the slopes of the different non-ideal signals differ as a function of signal power.
A combination of two or more of the foregoing techniques have also been used in the prior art for receiver control purposes.
It should be noted that instead of the absolute signal levels (e.g., the RSS or the total power at some node), the SIR, or the SNR, or, in a CDMA system, the Ec/Io can be utilized as well by the receiver control logic.